GB-PS No. 1.220,353 discloses a rotor cutter of the above type, which comprises a cutting member arranged on a vertical rotary shaft. The cutting member comprises a knife holder mounted at the lower end of the shaft, a plurality of knives with a shank extending downwards and outwards from said knife holder, and a blade formed integral with said shank and adapted to rotate in a plane perpendicular to the longitudinal axis of the rotary shaft. At the upper end the shank of the knife is pivotally connected to the knife holder by means of a pin. The longitudinal axis of the pin forms an acute angle with the rotary shaft. By such a rotor cutter, the shank of the knife is subjected to a large moment especially at its upper end due to the centrifugal force caused by the rotation of the cutting member. The shank of the knife must therefore be relatively thick in order to resist the above moment. On the other hand, the knife blade must be relatively thin in order to provide the best possible cutting result. An unfortunate result of the above contradicting conditions is that such a knife is relatively expensive to manufacture. The integral forming of the shank and the blade of the knife is furthermore encumbered with the drawback that the entire knife must be replaced when the blade is damaged, which in turn results in high operating expenses. The achievable cutting result suffers furthermore from the drawback that the entire knife swivels upwards and backwards when nothing but the blade hits an obstacle.
DE-PS No. 29 20 244 discloses a rotor cutter where the knife holder of the cutting member is a disk. On the bottom side of the disk, knives are rotatably arranged about an axis parallel to the axis of rotation of the cutting member, said knives projecting radially outwards from the disk when the cutting member is rotating. As a result the knives turn and clear possible obstacles such as a small stone or a stump. Experience has, however, shown that this type of cutting members involves a high risk of said cutting member and the driving mechanism causing said member to rotate being seriously damaged when the cutting member hits a large object such as a large stone or a large stump.